Thermoplastic resin foamed article

ABSTRACT

Disclosed is a thermoplastic resin foamed article, wherein the foamed article has two opposite surfaces, wherein the foamed article has at least a skin layer which defines one of the opposite surfaces and has a porosity of 0% or more but less than 1%, a lower-expansion layer which has a porosity of not less than 1% but less than 40% and is arranged so as to be adjacent to the skin layer, and a higher-expansion layer which has a porosity of not less than 40% but less than 100% and is arranged so as to be adjacent to the lower-expansion layer, wherein the skin layer, the lower-expansion layer and the higher-expansion layer are made of the same thermoplastic resin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.10/893,573, filed Jul. 19, 2004, and claims the benefit of priorityunder 35 U.S.C. §119 based on Japanese Application No. 2003-200308 filedJul. 23, 2003 and Japanese Application No. 2003-207277, filed Aug. 12,2003, the entire disclosures of all the aforesaid applications arehereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermoplastic foamed article.

2. Description of the Related Art

Heretofore, automotive parts, household electrical appliance parts andother industrial parts have been strongly requested to have light weightbut is superior in rigidity. As a material for forming such partsmeeting those requests, thermoplastic foamed articles are known.

As an example of the thermoplastic foamed articles, JP-A-2002-234046discloses a product comprising two layers including a skin layer and afoamed core layer. However, it still has plenty of room for improvementin rigidity.

JP-A-8-108440 discloses a foam board in which cells have a ratio oftheir size in the thickness direction to their size in a directionperpendicular to the thickness direction is from 2.5 to 10. However,although it is superior in cushion property and shock absorption, thereis a problem with it in that it has insufficient rigidity.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a thermoplastic resinfoamed article superior in rigidity.

A more specified object of the present invention is to provide athermoplastic resin foamed article which is superior in rigidity andalso is excellent in cushion property and shock absorption.

The present invention provides a thermoplastic resin foamed article,wherein the foamed article has two opposite surfaces, wherein the foamedarticle has at least a skin layer which defines one of the oppositesurfaces and has a porosity of 0% or more but less than 1%, alower-expansion layer which has a porosity of not less than 1% but lessthan 40% and is arranged so as to be adjacent to the skin layer, and ahigher-expansion layer which has a porosity of not less than 40% butless than 100% and is arranged so as to be adjacent to thelower-expansion layer, wherein the skin layer, the lower-expansion layerand the higher-expansion layer are made of the same thermoplastic resin.

One preferred embodiment of the present invention is a foamed articlewherein the higher-expansion layer has therein cells which have a ratioof D1 to D2, D1/D2, of from 1 to 4 wherein D1 denotes the length of thecells in the thickness direction of the higher-expansion layer and D2denotes the length of the cells in a direction perpendicular to thethickness direction.

Another preferred embodiment of the present invention is a foamedarticle wherein the higher-expansion layer has therein cells, whereincells located in a portion in the higher-expansion layer near thelower-expansion layer have a ratio of Da1 to Da2, Da1/Da2, of from 1 to4 where Da1 denotes the length of the cells in the thickness directionof the higher-expansion layer and Da2 denotes the length of the cells ina direction perpendicular to the thickness direction, and wherein cellslocated in a central portion in the higher-expansion layer have a ratioof Db1 to Db2, Db1/Db2, of more than 4 but not more than 10, where Db1denotes the length of the cells in the thickness direction of thehigher-expansion layer and Db2 denotes the length of the cells in adirection perpendicular to the thickness direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic enlarged sectional view along the thicknessdirection of a thermoplastic resin foamed article of the presentinvention.

FIG. 2 is a schematic enlarged sectional view of a portion near thecenter of a higher-expansion layer of a thermoplastic resin foamedarticle of the present invention.

FIG. 3 is a schematic enlarged sectional view along the thicknessdirection of a thermoplastic resin foamed article of the presentinvention.

FIG. 4 is a schematic enlarged sectional view of a portion near thecenter of a higher-expansion layer of a thermoplastic resin foamedarticle of the present invention.

FIG. 5 is a schematic enlarged sectional view of a thermoplastic resinfoamed article of the present invention, the article having a surfacedefined by a higher-expansion layer.

FIG. 6 is an outside view of a thermoplastic resin foamed articleproduced in the working examples.

In the drawings, each of the reference numerals has a meaning shownbelow:

1: skin layer, 2: lower-expansion layer, 3: higher-expansion layer, 4:cells of lower-expansion layer, 5: cells of higher-expansion layer, 5 a:cells located in higher-expansion layer near a lower-expansion layer, 5b: cells located near a central portion in higher-expansion layer, and11: gate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, the thermoplastic resin foamed article refersto a thermoplastic resin article having cells inside. In the followingdescription, a “thermoplastic resin foamed article” is referred simplyto as a “foamed article”.

As shown in FIG. 1, the foamed article of the present invention has twoopposite surfaces and the foamed article has at least a skin layer whichdefines one of the opposite surfaces and has a porosity of 0% or morebut less than 1%, a lower-expansion layer which has a porosity of notless than 1% but less than 40% and is arranged so as to be adjacent tothe skin layer, and a higher-expansion layer which has a porosity of notless than 40% but less than 100% and is arranged so as to be adjacent tothe lower-expansion layer, wherein the skin layer, the lower-expansionlayer and the higher-expansion layer are made of the same thermoplasticresin.

Moreover, in one preferred embodiment, as shown in FIG. 3, there is aspecific cell size distribution in the higher-expansion layer in thefoamed article of the present invention.

If, instead the combination of the skin layer (1) and thelower-expansion layer (2), only a skin layer having a thicknesscorresponding to the combined thickness of the skin layer (1) and thelower-expansion layer (2) is present, the light weight property, thecushion property and the shock absorption will become insufficient. Onthe other hand, if, instead the combination of the lower-expansion layer(2) and the higher-expansion layer (3), only a higher-expansion layerhaving a thickness corresponding to the combined thickness of thelower-expansion layer (2) and the higher-expansion layer (3) is present,the rigidity of the foamed article will decrease. The higher theporosity of the higher-expansion layer (3), the better for the purposesof reducing the weight and improving the rigidity, the cushion propertyand the shock absorption. However, in order to avoid an extremereduction in strength, the porosity is preferably 98% or less.

The foamed article of the present invention must have at least one skinlayer (1), at least one lower-expansion layer (2) and at least onehigher-expansion layer (3). However, it is desirable that two skinlayers (1), two lower-expansion layers (2) and one higher-expansionlayer (3) are formed in an arrangement: skin layer (1)/lower-expansionlayer (2)/higher-expansion layer (3)/lower-expansion layer (2)/skinlayer (1).

The measurement of the porosity is carried out based on a photograph,magnified by a scanning electron microscope (SEM), of a cross section ofthe foamed article. For a cut sample, proportions of areas occupied bycells found in a lower-expansion layer (2) and cells in ahigher-expansion layer (3) in a cross section along the thicknessdirection of the foamed article are determined by image analysis. Basedon the results, the porosity of the lower-expansion layer (2) and thatof the higher-expansion layer (3) can be calculated.

The cells may be either closed cells or open cells. Closed cells areadvantageous with respect to the shock absorption against a strongshock, whereas open cells are advantageous with respect to the cushionproperty and the shock absorption against a weak shock.

The shape and size of cells are not particularly restricted, but thelower-expansion layer (2) desirably has an average cell size of 100 μmor less and the higher-expansion layer (3) desirably has an average cellsize of from 30 μm to 3000 μm.

The cell size is measured based on a photograph, magnified by a scanningelectron microscope (SEM), of a cross section of the foamed article. Fora cut sample, a minimum size and a maximum size of cells (4) and (5)found in a lower-expansion layer (2) and a higher-expansion layer (3) inthe cross section along the thickness direction of the foamed articleare measured. The average of the measurements is used as the cell size.

The expansion ratio of the foamed article is defined as a value obtainedby dividing the specific gravity of the material constituting the foamedarticle in an unfoamed state by the specific gravity of the foamedarticle and is preferably 1.25 or more, more preferably 1.5 or more, andstill more preferably 3 or more. When great importance is attached tothe cushion property and the shock absorption, the foamed articledesirably has an expansion ratio of 4 or more.

The thicknesses of the skin layer (1), the lower-expansion layer (2) andthe higher-expansion layer are not particularly restricted and may bedetermined appropriately depending on the application, usage purpose andrequired performance of the foamed article. For a foamed article whichis light in weight but high in rigidity, it is desirable that the skinlayer (1) have a thickness ranging from 0.05 to 0.7 mm, that thelower-expansion layer (2) have a thickness ranging from 0.05 to 0.7 mm,and that the higher-expansion layer (3) have a thickness ranging from0.1 to 50 mm. When great importance is attached to the cushion propertyand the shock absorption, it is desirable that the skin layer (1) have athickness ranging from 0.05 to 0.7 mm, that the lower-expansion layer(2) have a thickness ranging from 0.05 to 0.7 mm, and that thehigher-expansion layer (3) have a thickness ranging from 3 to 80 mm.

The foamed article of the present invention is not required that thewhole foamed article be constituted of the aforementioned three types oflayers, namely the skin layer, the lower-expansion layer and thehigher-expansion layer. It is only required that a desired portion in afoamed article be constituted of the three types of layers, namely theskin layer, the lower-expansion layer and the higher-expansion layer.

In one embodiment of the present invention, it is desirable that, asshown in FIG. 2, cells in the higher-expansion layer have a ratio of D1to D2, D1/D2, of from 1 to 4 wherein D1 denotes the length of the cellsin the thickness direction of the higher-expansion layer and D2 denotesthe length of the cells in a direction perpendicular to the thicknessdirection.

If the value of D1/D2 is smaller than 1, the foamed article hastendencies to exhibit a reduced resistance against bending deformationand to exhibit a reduced resilience against compressive deformation.

If the value of D1/D2 is larger than 4, the foamed article has atendency to have a reduced rigidity to buckle from bending deformation.

D1 and D2 are determined using a photograph, magnified by a scanningelectron microscope (SEM), of a central portion of the higher-expansionlayer with respect to its thickness direction in a cross section of thefoamed article, as shown in FIG. 2. For at least ten cells chosen atrandom from the cells located in the central portion of thehigher-expansion layer, each cell is measured for a maximum size in thethickness direction and a maximum size in a direction perpendicular tothe thickness direction. The average of the measurements in thethickness direction of the chosen cells and the average of themeasurements in the direction perpendicular to the thickness directionof the chosen cells are used as D1 and D2, respectively.

In a foamed article shown in FIG. 3, if cells (5 a) located in a portionof the higher-expansion layer near the lower-expansion layer has a valueof Da1/Da2 smaller than 1, the foamed article has a tendency to exhibita reduced resistance against bending deformation and to exhibit areduced resilience against compressive deformation. On the other hand,if the value of Da1/Da2 is larger than 4, the foamed article has atendency to have a reduced rigidity to buckle from bending deformation.By the “portion of the higher-expansion layer near the lower-expansionlayer” is meant a “region of the higher-expansion layer extending within10% of the thickness of the higher-expansion layer from the boundarybetween the lower-expansion layer and the higher-expansion layer”.

In order for the foamed article to have an excellent cushion property,an excellent shock absorption and an excellent resistance againstcompressive deformation simultaneously, it is desirable that cells (5 b)in the central portion of the higher-expansion layer have a Db1/Db2ratio of more than 4 but not more than 10.

If the cells (5 b) in the central portion of the higher-expansion layerhave a Db1/Db2 ratio of 4 or less, the foamed article has a tendency todeteriorate with respect to the cushion property and the shockabsorption. On the other hand, if the value of Db1/Db2 is larger than10, the foamed article has a tendency to readily suffer compressivedeformation from an external force.

Da1 and Da2 are determined using a photograph, magnified by a scanningelectron microscope (SEM), of a section of the foamed article. Using themagnified photograph, each cell contained in a region sized 0.5 mm by0.5 mm adjacent to the lower-expansion layer is measured for a maximumsize in the thickness direction and a maximum size in a directionperpendicular to the thickness direction. The average of themeasurements in the thickness direction of the chosen cells and theaverage of the measurements in the direction perpendicular to thethickness direction of the chosen cells are used as Da1 and Da2,respectively.

Db1 and Db2 are determined using a photograph of a central portion inthe higher-expansion layer with respect to its thickness direction in asection of the foamed article, as shown in FIG. 4, magnified by ascanning electron microscope (SEM). For at least ten cells chosen atrandom from the cells located in the central portion in thehigher-expansion layer, each cell is measured for a maximum size in thethickness direction and a maximum size in a direction perpendicular tothe thickness direction. The average of the measurements in thethickness direction of the chosen cells and the average of themeasurements in the direction perpendicular to the thickness directionof the chosen cells are used as Db1 and Db2, respectively.

The absolute thickness of the portion in the higher-expansion layer (3)which portion is located near the lower-expansion layer and containscells having a Da1/Da2 ratio of from 1 to 4 is not particularlyrestricted and may be determined appropriately depending on theapplication, usage purpose and required performance of the foamedarticle. In order to achieve good cushion property and good shockabsorption, the thickness of the portion containing cells having aDa1/Da2 ratio of from 1 to 4 is preferably within the range of 0.3 mm to10 mm.

The thermoplastic resin for use in the present invention is notparticularly restricted and various types of thermoplastic resin may beemployed. Examples of the thermoplastic resin include olefin-basedresin, styrene-based resin, acrylic resin, amide resin, olefin-basedthermoplastic elastomer and styrene-based thermoplastic elastomer. Amongthese thermoplastic resins, olefin-based resin and olefin-basedthermoplastic elastomer are preferably employed.

In the present invention, the olefin-base resin includes resins havingolefin-derived repeating units in an amount of 50% by weight or more. Itincludes homopolymers of α-olefins having 20 or less carbon atoms, e.g.ethylene, propylene, butene-1, pentene-1, hexene-1, 3-methylbutene-1 and4-methylpentene-1; copolymers obtained by copolymerization of at leasttwo kinds of monomers selected from the aforementioned α-olefins; andcopolymers of those α-olefins and other unsaturated monomerscopolymerizable with those α-olefins.

Examples of the unsaturated monomers copolymerizable with α-olefinsinclude unsaturated carboxylic acid such as acrylic acid and methacrylicacid; derivatives of alkyl esters of unsaturated carboxylic acids, e.g.methyl (meth)acrylate, 2-ethylhexyl acrylate, ethyl (meth)acrylate andbutyl (meth)acrylate; unsaturated dicarboxylic acids or theiranhydrides, e.g. fumaric acid, maleic acid, maleic anhydride anditaconic acid; derivatives of unsaturated carboxylic acids orunsaturated dicarboxylic acids, e.g. acrylamide,N-(hydroxymethyl)acrylamide, glycidyl (meth)acrylate, acrylonitrile,methacrylonitrile, mono- or diethyl ester of maleic acid,N-phenylmaleimide and N,N′-metaphenylene bismaleimide.

As the olefin-based resin to be used in the present invention,polypropylene resin is preferred. Examples of the polypropylene resininclude homopolymers of propylene and copolymers of propylene with atleast one kind of monomer selected from the group consisting of ethyleneand α-olefins having 4 to 12 carbon atoms. These homopolymers andcopolymers may be used alone or in combination. Examples of theα-olefins having 4 to 12 carbon atoms include 1-butene,4-methyl-1-pentene, 1-hexene and 1-octene. The copolymers of propylenewith at least one kind of monomer selected from ethylene and α-olefinshaving 4 to 12 carbon atoms desirably are copolymers containing at least50% by weight of repeating units derived from propylene, which mayhenceforth be referred to as “propylene units,” based on 100% by weightof the copolymers.

The flexibility and impact resistance of the copolymers can becontrolled by choosing the amount of repeating units derived fromethylene or α-olefins having from 4 to 12 carbon atoms.

If a copolymer has two or more kinds of repeating units other thanpropylene units, the total amounts of the repeating units other thanpropylene units is preferably not more than 35% by weight.

Specific examples of the polypropylene resin include (i) homopolymers ofpropylene, (ii) random copolymers of propylene and ethylene, (iii)random copolymers of propylene and α-olefin, (iv) random copolymers ofpropylene, ethylene and α-olefin, and (v) block copolymers propylene andethylene.

The polypropylene resin preferably has a melt flow rate (MFR), measuredin accordance with JIS K 6758, of from 1 to 100 g/10 min. From theviewpoint of molding processability, it is more preferably 5 g/min ormore; and still more preferably 8 g/10 min.

Polypropylene resins which are particularly preferred in the presentinvention preferably contain a polymer in which an ultra high molecularweight component is introduced, the polymer being prepared by producing,in a first stage, a polypropylene polymer (I) having an intrinsicviscosity of 5 dl/g or more, which is an ultra high molecular weightcomponent, by polymerizing monomers composed mainly of propylene andthen continuously producing, in a second or later stage, a polypropylenepolymer (II) having an intrinsic viscosity less than 3 dl/g bypolymerizing monomers composed mainly of propylene; or a polypropylenepolymer ordinarily having a branching structure where a polypropyleneresin has long-chain branches mainly at its ends, the polymer beingobtained by a method described in JP-A-62-121740 comprising introducinglong-chain branches through crosslinking using low-level radiation, amethod comprising reacting a polypropylene polymer, aradically-polymerizable monomers and a radical initiator together, amethod comprising mixing a polypropylene resin and a radical initiatortogether under a temperature condition where breakage of primary chainsof the resin does not occur preferentially.

In addition, a resin other than olefin-based resin may be added to theolefin-based resin unless the object of the present invention isdisturbed. Examples of the resin other than olefin-based resin includestyrene-based resin, acrylic resin, amide resin and styrene-basedelastomer such as hydrogenation products of styrene-butadiene diblockcopolymers, hydrogenation products of styrene-butadiene-styrene triblockcopolymers and hydrogenation products of styrene-isoprene-styrenetriblock copolymers, and mixtures thereof. These may be addedappropriately depending on application.

The olefin-based thermoplastic elastomer refers to a combination of anolefin copolymer rubber and an olefin polymer in any desired weightratio. The olefin copolymer rubber may be present in an uncrosslinked,partially crosslinked or completely crosslinked state in thethermoplastic elastomer. Preferably used is a crosslinked olefin-basedthermoplastic elastomer in which an olefin copolymer rubber is presentin a partially or completely crosslinked state.

The olefin copolymer rubber is an amorphous random elastic copolymercomposed mainly of olefin. Examples thereof include ethylene-propylenecopolymer rubber, ethylene-α-olefin copolymer rubber,ethylene-propylene-α-olefin copolymer rubber,ethylene-propylene-nonconjugated diene copolymer rubber,ethylene-butene-1-nonconjugated diene copolymer rubber, andpropylene-butene-1 copolymer rubber. Of these rubbers,ethylene-propylene-nonconjugated diene copolymer rubber andethylene-propylene copolymer rubber are preferred. Examples of thenonconjugated diene include dicyclopentadiene, 1,4-hexadiene,cyclooctadiene, methylene norbornen and ethylidene norbornene. Inparticular, ethylidene norbornene is preferred. As the olefin copolymerrubber, particularly preferred is an ethylene-propylene-ethylidenenorbornene copolymer rubber having a propylene unit content of from 10to 55% by weight, preferably from 20 to 40% by weight, and an ethylidenenorbornene unit content of from 1 to 30% by weight, preferably from 3 to20% by weight.

The olefin copolymer rubber can be produced by known methods. Examplesof the catalyst to be used include Ziegler-Natta catalysts andmetallocene-type homogeneous catalysts.

The olefin-based copolymer is a polymer composed mainly (preferably 50%by weight or more) of olefin. Examples thereof include propylenehomopolymers and propylene-α-olefin copolymer. These may be used aloneor in combination. Examples of the α-olefin include ethylene, 1-butene,1-pentene, 3-methyl-1-butene, 1-hexene, 1-decene, 3-methyl-1-pentene,4-methyl-1-pentene and 1-octene.

In the present invention, the above-mentioned thermoplastic resin may beused alone or two or more kinds of resins may be used in combinationdepending on the purpose. For example, use of a hard resin such asolefin-based resin will result in a foamed article which is lighter andsuperior in rigidity in comparison to conventional injection moldedarticles or conventional foamed articles. On the other hand, use of asoft resin such as olefin-based thermoplastic elastomer will result in afoamed article which is lighter and is well-balanced with respect torigidity and soft feeling in comparison to conventional injection moldedarticles or conventional foamed articles.

When an olefin-based resin and an olefin-based thermoplastic elastomerare used in combination in any desired ratio, a foamed article can beobtained which is balanced with respect to rigidity and soft feelingdepending on the purpose.

The thermoplastic resin used in the present invention may containinorganic filler such as talc, mica, clay, calcium carbonate, aluminiumhydroxide, magnesium hydroxide, wollastonite, barium sulfate, glassfiber, carbon fiber, silica, calcium silicate, potassium titanate andwollastonite.

Moreover, the thermoplastic resin used in the present invention maycontain various kinds of additives. Examples of such additives includeantioxidants such as phenol-type antioxidant, organic phosphite-typeantioxidant, organic phosphorus-type antioxidants; heat stabilizers suchas hindered amine-type heat stabilizers; UV absorbers such asbenzophenone-type UV absorbers, benzotriazole type UV absorbers andbenzoate type UV absorbers; antistatic agents such as nonionicantistatic agents, cationic antistatic agents and anionic antistaticagents; dispersing agents such as bisamide-type dispersing agents,wax-type dispersing agents and organometal salt-type dispersing agents;chlorine scavengers; lubricants such as amide-type lubricants, wax-typelubricants, organometal salt-type lubricants and ester-type lubricants;decomposers such as oxide-type decomposers and hydrotalcite-typedecomposers; metal deactivators such as hydrazine-type metaldeactivators and amine-type metal deactivators; flame retardants such asbromine-containing organic flame retardants, phosphoric acid-type flameretardants, antimony trioxide, magnesium hydroxide and red phosphorus;organic pigments; inorganic pigments; organic filler; inorganic ororganic antibacterial agents such as metal ion-type antibacterialagents; and nucleating agents such as organophosphoric acid nucleatingagents and sorbitol compounds.

As a method for producing the foamed article of the present invention,known methods such as injection expansion molding, press expansionmolding, extrusion expansion molding and stamping expansion molding. Thefoamed article is preferably produced by injection expansion molding.

In the injection expansion molding, a thermoplastic resin containing afoaming agent is charged into a mold cavity of an injection moldingmachine, and then the volume of the mold cavity is maintained for awhile. Thus, a skin layer and a lower-expansion layer having a porosityof not less than 1% but less than 40% are formed. After that, at least apart of the mold cavity is enlarged. Through this operation, the resinlocated in the central portion with respect to the thickness directionof the cavity is further expanded to have a porosity of not less than40%. Thus, a higher-expansion layer is formed. Subsequently, the foamedresin is cooled to solidify. The foamed article obtained in this way isused for various applications because it has a favorable foam structurecontaining no coarse cells.

Examples of the method for forming a lower-expansion layer having aporosity of not less than 1% but less than 40% include a first methodwhich comprises packing a foaming agent-containing resin fully into amold cavity, allowing the resin to expand by a volume corresponding toits shrinkage from its cooling by utilizing expansion of the gas of thefoaming agent, thereby filling the whole mold cavity with the expandedresin; a second method comprising injecting a foaming agent-containingresin of a volume smaller than the capacity of a mold cavity into themold cavity, and allowing the resin to expand by utilizing expansion ofthe gas of the foaming agent, thereby filling the whole mold cavity withthe expanded resin; and a third method comprising injecting a foamingagent-containing resin into a mold cavity to fully packing it, enlargingat least a part of the mold cavity so as to allow the resin to have aporosity of not less than 1% but less than 40%, thereby allowing theresin to expand utilizing expansion of the gas of the foaming agent, andthereby filling the whole mold cavity with the expanded resin. The firstmethod is preferred.

Examples of concrete methods for forming a higher-expansion layer byenlarging at least a part of the mold cavity to allow the resin locatedin the central portion with respect to the thickness direction of thecavity, thereby further expanding the resin to have a porosity of notless than 40% include: a method comprising enlarging the whole cavity byretracting the mold cavity surface, a method comprising partly and/orentirely enlarging the cavity using a slide core, and a methodcomprising a combination of the above methods.

The expansion molding may be carried out in combination with any methodsuch as gas assist molding, melt core molding, insert molding andtwo-color molding.

The shape of the foamed article is not particularly restricted.

The foaming agent for use in the present invention is not particularlyrestricted and conventional foaming agents including chemical foamingagents and physical foaming agents may be employed.

The chemical foaming agent may be any substance which does not decomposeat temperatures lower than the melting temperature of the thermoplasticresin but decomposes or reacts at temperatures not lower than themolting temperature of the thermoplastic resin. The chemical foamingagent may be either an inorganic compound or an organic compound. Two ormore agents may be used in combination.

Examples of the inorganic compound include hydrogencarbonates such assodium hydrogencarbonate, and ammonium carbonate.

Examples of the organic compound include polycarboxylic acids, azocompounds, sulfone hydrazide compounds, nitroso compounds,p-toluenesulfonyl semicarbazide and isocyanate compounds.

The polycarboxylic acids include citric acid, oxalic acid, fumaric acidand phthalic acid.

The azo compounds include azodicarbonamide (ADCA).

The sulfone hydrazide compounds include p-methylurethane benzenesulfonylhydrazide, 2,4-toluene disulfonyl hydrazide and4,4′-oxybisbenzenesulfonyl hydrazide.

The nitroso compounds include dinitrosopentamethylene tetramine (DPT).

Examples of the physical foaming agents include inert gas such asnitrogen and carbon dioxide, and volatile organic compounds other thanfreon-type organic compounds, such as butane and pentane. Two or morephysical foaming agents may be used in combination. Moreover, a chemicalfoaming agent and a physical foaming agent may be employed incombination.

The foaming agent to be used in the present invention preferably isinert gas. The inert gas preferably is an inorganic substance whichexhibits no reactivity to the resin to be employed and has noprobability of degrading the resin and which is in a gaseous form underambient temperature and pressure conditions. Examples of inert gasinclude carbon dioxide, nitrogen, argon, neon, helium and oxygen. Thesemay be used either alone or in combination. Among them, carbon dioxide,nitrogen and a mixture thereof are preferably employed because they areinexpensive and are of high safety.

Use of supercritical inert gas as a foaming agent is preferred becauseit exhibits a high solubility to resin and can be diffused evenly in theresin within a short time and therefore it has an effect of increasingthe number of cells to be formed to yield a foamed article having a goodfoamed cell structure. Moreover, because the supercritical inert gas asa foaming agent can generate a high foaming pressure, the whole moldcavity can be filled with an expanded resin at a time just after thecompletion of injection of the foaming-agent containing resin into themold cavity. It, therefore, is preferably employed for the method whichcomprises allowing a resin to expand by the a volume corresponding tothe shrinkage of the resin from its cooling, by utilizing expansion ofthe gas of the foaming agent.

As a method for mixing thermoplastic resin and inert gas, a method inwhich an inert gas in a supercritical state as a foaming agent ischarged into a cylinder of an injection molding machine and then amolten resin and the inert gas are mixed, dispersed and dissolved ispreferred because a foamed article is foamed evenly overall and amolding cycle can be shortened. As a supercritical inert gas,supercritical nitrogen, supercritical carbon dioxide and a supercriticalmixture of nitrogen and carbon dioxide are preferably employed.

The foamed article of the present invention can be integrated in layerwith a substrate by injection molding, injection compression molding,adhering or the like to form a laminate.

The foamed article in the laminate has a covering material on itssurface on which the substrate is not present.

Moreover, the foamed article of the present invention may be combinedwith a covering material to form a decorated foamed article.

As such a covering material, known covering materials may be employed.Specific examples of covering materials include woven fabric, nonwovenfabric, knitted fabric, films and sheets made of thermoplastic resin orthermoplastic elastomer. Moreover, also available are composite coveringmaterials in which nonfoamed sheets of polyurethane, rubber,thermoplastic elastomer or the like are integrally laminated with thosecovering materials.

The covering materials may be provided with a cushion layer. Thematerial for constituting such a cushion layer may include polyurethanefoam, EVA foam, polypropylene foam and polyethylene foam.

Moreover, it is also possible to provide a covering material with abacking layer which serves as a protection layer. Examples of thebacking layer include woven fabric, nonwoven fabric, knitted fabric,films and sheets made of thermoplastic resin or thermoplastic elastomer

The covering material to be applied for the present invention preferablyhas air permeability. Examples thereof include woven fabric, knittedfabric, nonwoven fabric similar to those previously mentioned andmaterials prepared from sheets or films of thermoplastic resin orthermoplastic elastomer by making pores in the sheets or films to impartair permeability to them. When the covering material has a multilayerstructure, not all the layers constituting the covering material arerequired to have air permeability. It is only required that at least thelayer which is to be laminated to a foamed article of the presentinvention have air permeability.

The foamed article of the present invention may be used in aconventional molding method to form a laminate having a substratecombined with the foamed article. Examples of conventional moldingmethods include insert molding and two-color molding.

In the insert molding, a foamed article of the present invention isproduced in advance and then is placed in a cavity of a mold forinjection molding. After that, a thermoplastic resin for forming asubstrate is injection molded. Thus, a laminate in which the foamedarticle is firmly bonded to the substrate is obtained.

In addition, a laminate in which a foamed article is firmly bonded to asubstrate can also be obtained by molding a thermoplastic resin to forma substrate, placing the substrate in a mold cavity, and then expansionmolding a foamed article of the present invention in the mold cavity.

In the two-colormolding, a thermoplastic resin for forming a substrateis injected, and then a foamed article of the present invention isexpansion molded. Thus, a laminate in which a substrate made of thethermoplastic resin is bonded firmly to the foamed article of thepresent invention is obtained

As a thermoplastic resin for forming a substrate, various kinds of resinmay be used. Particularly, propylene-based resin is preferably employed.Examples of the propylene-based resin include propylene homopolymer,propylene-α-olefin random copolymer and propylene-ethylene blockcopolymer. These may be used alone or in combination. Moreover,materials resulting from mixing thermoplastic elastomer, rubber orvarious types of inorganic filler with propylene-based resin are alsopreferred.

When insert molding is carried out using a laminate prepared byexpansion molding a foamed article while placing a covering material ina mold cavity, a new laminate can be obtained which has a structure:covering material/foamed article/substrate. Alternatively, a laminatehaving a structure of covering material/foamed article/substrate can beobtained also by a method in which a covering material and a substrateare placed in a mold cavity in advance and then a foamed article of thepresent invention is expansion molded between the covering material andthe substrate.

In the present invention, it is desirable that the foamed article orlaminate have a surface defined by a higher-expansion layer because notonly cushion property and shock absorption but also sound absorptionproperty of the foamed article or laminate are improved.

The method for producing the foamed article or laminate having ahigher-expansion surface is not particularly restricted if the producthas the specific foam structure of the present invention. One example isa method in which a foamed article or laminate obtained by theaforementioned injection expansion molding or the like is cut in thehigher-expansion layer.

The foamed article of the present invention has at least three kinds oflayers, namely a skin layer, a lower-expansion layer and ahigher-expansion layer. It, therefore, is light in weight and also issuperior in rigidity. Particularly, a foamed article whosehigher-expansion layer has the aforementioned special foam structure isexcellent in cushion property and shock absorption and is also superiorin rigidity. Therefore, such foamed articles can be used suitably forvarious applications such as automotive parts, household electricalappliance parts and other industrial parts.

EXAMPLES

The present invention will be described more concretely below byreference to examples. However, the invention is not limited to theseexamples.

Evaluation Method

Melt Flow Rate (MFR)

The melt flow rate was determined, according to JIS K 7210, underconditions including a temperature of 230° C. and a load of 2.16 kgf forresins composed mainly of repeating units derived from propylene.

Total Expansion Ratio

The total expansion ratio of a foamed article was indicated by a valueobtained by dividing a specific gravity of the foamed article measuredusing a hydrometer (an electronic hydrometer EW-200SG available fromMirage Trading Co., Ltd.) by a specific gravity of an unfoamed material.

Thickness and Porosity of Skin Layer, Low-Expansion Layer andHigh-Expansion Layer

A foamed article was cut. The formed cross section was observed by ascanning electron microscope and the condition of cells were judged.Moreover, the thickness measurement was conducted for a skin layer, alower-expansion layer and a higher-expansion layer. The porosities ofthe lower-expansion layer and the higher-expansion layer were determinedthrough image analysis of the layers using image processing softwareavailable from Nano System Corp. “Nona Hunter NS 2K-Pro”.

Rigidity

A test piece sized 50 mm by 150 mm was cut from a foamed article and washeld at its both ends at a span of 100 mm. To the center of the testpiece, a load was applied so that the test piece deflects at a rate of50 mm/min. Thus, a load-deflection curve was produced. Using a slope ofthe initial straight part of the curve, a load (N/cm) required forgenerating 1 cm deflection was calculated and it was used as a measureof the rigidity of the foamed article.

D1/D2, Da1/Da2 and Db1/Db2

A foamed article was cut and a section was observed by a scanningelectron microscope. From a magnified photograph of a higher-expansionlayer, D1, D2, Da1, Da2, Db1 and Db2 was determined and then D1/D2,Da1/Da2 and Db1/Db2 were calculated.

Cushion Property

A foamed article was pushed with a finger. The cushion property wasexamined by the touch feeling.

Example 1

As a thermoplastic resin, polypropylene AZ161C (manufactured by SumitomoMitsui Polyolefin Co., Ltd., MFR 30 g/10 min) was used. Expansionmolding was carried out using an ES2550/400HL-MuCell (clamping force 400ton) manufactured by ENGEL as an injection molding machine and a moldhaving a box-shaped molding section having dimensions of 290 mm by 370mm, 45 mm in height and 2 mm in thickness (gate structure: bubble gatelocated in the central portion of a molded article) as shown in FIG. 2.As a foaming agent, supercritical nitrogen was used. It was suppliedinto the cylinder of the molding machine while being compressed to 20MPa. The amount of the foaming agent supplied was 1.2%. A mixture of thethermoplastic resin and a foaming agent was injected into the mold at amolding temperature of 200° C. and a mold temperature of 60° C. so as tofully pack the mold. Then, the molten resin was allowed to expand for7.5 seconds without being applied with pressure and a expanded resin ofa lower-expansion layer was cooled to solidify. Then, the cavity wall ofthe mold was retracted by 1.95 mm to enlarge the cavity capacity. Thus,the central portion of the resin with respect to the cavity thicknessdirection was allowed to expand. The expanded resin was cooled tosolidify. Thus, a foamed article was obtained and then evaluated. Theresults are shown in Table 1.

Comparative Example 1

A molten thermoplastic resin containing a foaming agent was injectedinto a mold so as to fully pack the mold. A thermoplastic resin foamedarticle was obtained and then evaluated in the same manner as Example 1except that the cavity wall of the mold was retracted by 1.95 mm in 0.5second from the injection so as not to form a lower-expansion layer inthe mold. The article had no lower-expansion layer and had a reducedrigidity in comparison to Example 1. The results are shown in Table 1.

TABLE 1 Comparative Example 1 Example 1 Total expansion ratio 2.0 2.0Thickness (mm) Skin layer 0.3 0.2 Lower-expansion layer 0.2 0Higher-expansion layer 3.0 3.6 Porosity (%) Lower-expansion layer 15Higher-expansion layer 65 56 D1/D2 2.2 1.5 Rigidity (N/cm) 128 95

Example 2

As a thermoplastic resin, employed was a polypropylene resin obtained bymixing polypropylene AZ161C (manufactured by Sumitomo Mitsui PolyolefinCo., Ltd., MFR 30 g/10 min) and a long-chain-branching homopolypropylene“PF814” (manufactured by BASELL, MFR 2.2 g/10 min) in a ratio of 80/20.Expansion molding was carried out using an ES2550/400HL-MuCell (clampingforce 400 ton) manufactured by ENGEL as an injection molding machine anda mold having a box-shaped molding section having dimensions of 290 mmby 370 mm, 45 mm in height and 2 mm in thickness (gate structure: bubblegate located in the central portion of a molded article) as shown inFIG. 4. As a foaming agent, supercritical nitrogen was used. It wassupplied into the cylinder of the molding machine while being compressedto 20 MPa. The amount of the foaming agent supplied was 1.5%. A mixtureof the thermoplastic resin and a foaming agent was injected into themold at a molding temperature of 200° C. and a mold temperature of 60°C. so as to fully pack the mold. Then, the molten resin was allowed toexpand for 7.5 seconds without being applied with pressure and aexpanded resin of a lower-expansion layer was cooled to solidify. Then,the cavity wall of the mold was retracted by 1.95 mm to enlarge thecavity capacity. Thus, the central portion of the resin with respect tothe cavity thickness direction was allowed to expand. The expanded resinwas cooled to solidify. Thus, a foamed article was obtained and thenevaluated. The results are shown in Table 2.

TABLE 2 Example 2 Total expansion ratio 10 Thickness (mm) Skin layer 0.3Lower-expansion layer 0.2 Higher-expansion layer 18 Da1/Da2 1.6 Db1/Db26.5 Cushion property Good Rigidity Good

1. A thermoplastic resin foamed article, wherein the foamed article hastwo opposite surfaces, wherein the foamed article has at least a skinlayer which defines one of the opposite surfaces and has a porosity of0% or more but less than 1%, a lower-expansion layer which has aporosity of not less than 1% but less than 40% and is arranged so as tobe adjacent to the skin layer, and a higher-expansion layer which has aporosity of not less than 40% but less than 100% and is arranged so asto be adjacent to the lower-expansion layer, wherein the skin layer, thelower-expansion layer and the higher-expansion layer are made of thesame thermoplastic resin, wherein the higher-expansion layer has thereincells which have a ratio of D1 to D2 of from 2 to 4 wherein D1 denotesthe length of the cells in the thickness direction of thehigher-expansion layer and D2 denotes the length of the cells in adirection perpendicular to the thickness direction.
 2. The foamedarticle according to claim 1, wherein the thermoplastic resin is anolefin-based resin or an olefin-based thermoplastic elastomer.
 3. Thefoamed article according to claim 1, wherein the foamed article isobtainable by a method comprising the steps: providing a mold havingtherein a cavity whose capacity is changeable, charging thermoplasticresin containing a foaming agent into a the cavity, holding the cavityto have a predetermined volume, thereby forming a skin layer having aporosity of 0% or more but less than 1% and a lower-expansion layerhaving a porosity of not less than 1% but less than 40%, enlarging thecavity to further expand a part of the lower-expansion layer, therebyforming a higher-expansion layer having a porosity of not less than 40%but less than 100%.
 4. The foamed article according to claim 3, whereinthe foaming agent is supercritical carbon dioxide, supercriticalnitrogen or a supercritical mixture of carbon dioxide and nitrogen.
 5. Alaminate in which the foamed article according to claim 1 is integrallylaminated on a substrate.
 6. The laminate according to claim 5, whereinthe foamed article has a covering material on its surface on which thesubstrate is not present.
 7. A decorated foamed article comprising thefoamed article according to claim 1 and a covering material integrallylaminated on the foamed article.